Process for the preparation of flexible circuits

ABSTRACT

Process for the preparation of flexible circuits formed from flexible photohardenable elements having at least one organic elastomeric polymeric binder which comprises 
     (a) exposing imagewise said element, 
     (b) applying particulate conductive metal, e.g., nickel, copper, to the unexposed image areas, optionally 
     (c) heating the metal bearing areas, then 
     (d) removing excess metal particles, optionally 
     (e) fixing the particulate metal to the layer by heating, exposing to UV light or mechanical embedding, and 
     (f) plating electrolessly or soldering the metal containing areas. The process can be operated continuously to prepare single and double layer flexible printed circuits, membrane switches, etc.

This is a divisional of application Ser. No. 304,107 filed Sept. 21,1981 now U.S. Pat. No. 4,411,980.

TECHNICAL FIELD

This invention relates to a process for the preparation of printedcircuits including multilayer printed circuits. More particularly thisinvention relates to a process for the preparation of flexible printedcircuits using flexible photohardenable elements.

BACKGROUND ART

The preparation of printed circuits, including multilayer printedcircuits is known. Powdered materials such as particulate metals havebeen applied to surfaces having imagewise tacky and nontacky areas by anumber of toning methods to produce printed circuit patterns.Representative methods are disclosed in U.S. Pat. Nos. 3,060,024;3,649,268; 4,054,479, 4,054,483, 4,517,407 and 4,234,626. After theparticulate metal is applied to the tacky image areas and unwantedparticles are removed, e.g., mechanically, from the nontacky imageareas, the circuit is formed by one of several additive techniquesincluding fusion of metal particles, electroless plating,electroplating, etc. The printed circuits formed by these additiveprocesses are useful, but the processes have certain disadvantages.While the printed circuits may possess some flexibility, it is necessaryin the preparation of some products, e.g., electronic switches, etc. toprovide highly flexible printed circuits which are capable of being bentcontinuously over a small radius. Electronic switches can be prepared byscreen printing the circuitry onto flexible film supports by means ofelectrically conductive ink. Such printing must be done uniformly in thecorrect position for each circuit and then dried properly. The processrequires skillful printers and special printing equipment and dryers.The types of inks used for preparing electronic switches do not alwaysdry uniformly to give reproducible conductivity and the adhesion of theink to its substrate is sometimes insufficient.

An object of this invention is to provide a process, including acontinuous process, whereby flexible printed circuits can be prepared byadditive procedures such as electroless plating and soldering, usingflexible photohardenable elements which are capable of being bent over asmall radius; the photohardenable composition being capable of survivingthe plating and soldering and providing reliable conductivity andadhesion. Another object of this invention is to provide a process forthe preparation of electronic or membrane switches using flexiblephotohardenable elements and additive techniques for the formation ofthe circuits.

BRIEF DESCRIPTION OF DRAWING

In the accompanying FIGURE of the drawing forming a material part ofthis disclosure the FIGURE is an elevation with parts in section showingthe various parts of an automated apparatus for practicing thisinvention wherein two flexible printed circuits are prepared, placed inregister face to face and laminated to each other.

DISCLOSURE OF INVENTION

In accordance with this invention there is provided a process for thepreparation of flexible circuits formed from a flexible photohardenableelement which comprises:

(a) exposing imagewise to actinic radiation the flexible photohardenableelement comprising, in order,

(1) a flexible support,

(2) a dry, tacky photohardenable layer comprising a photohardenablecomposition, optionally including at least one organic elastomericpolymeric binder compound, and, optionally,

(3) a removable cover sheet;

(b) removing the cover sheet, if present, and applying particulate metalto the unexposed image areas of the photohardenable layer; optionally

(c) heating the element to a temperature in the range of 25° to 80° C.;then

(d) removing excess particulate metal; optionally,

(e) fixing or binding the particulate metal to the photohardenable layerby heating, curing by exposing to actinic radiation, embeddingmechanically and combinations thereof; and

(f) plating electrolessly or soldering the particulate metal bearing ormetal containing areas thereby forming an electrically conductivecircuit.

Referring to the drawing the FIGURE shows the basic steps of anembodiment of the invention wherein flexible circuits are formed from aflexible photohardenable element which comprises, in order,

(1) a flexible support,

(2) a dry, tacky photohardenable layer comprising a photopolymerizable,photocrosslinkable or photodimerizable composition including at leastone organic elastomeric polymeric binder compound and a photoinitiatoror photoinitiator system, and, optionally but preferably,

(3) a removable cover sheet. Alternatively an overcoat layer, e.g.,polyvinyl alcohol, etc. can be present over the photohardenable layer inplace of the cover sheet. The overcoat layer is removed by aqueouswashout.

The term "flexible circuit" as used herein means a circuit wherein a0.001 inch (0.025 mm) thick layer laminate consisting of photohardenedmaterial and circuit material can be repeatedly bent around a structurehaving a diameter of 0.6 inch (15.24 mm) or less and preferably 0.375inch (9.53 mm) without breaking, cracking or delaminating. For circuitshaving layer laminates with thicknesses different from 0.001 inch (0.025mm), the above structure diameter criteria would be increased ordecreased proportionately with the proviso that the minimum diameter ofthe structure would always be greater than 0.25 inch (6.35 mm). Thus fora circuit layer laminate of 0.010 inch (0.254 mm) the structure wouldhave a diameter of 6 inches (15.24 cm) or less and preferably 3.75inches (9.53 cm). It is understood that the flexible support should haveflexibility at least comparable to the layer laminate. The terms"photohardenable" and "photopolymerizable" as used herein refer tosystems in which the molecular weight of at least one component of thephotosensitive material is increased by exposure to actinic radiationsufficiently to result in a change in the rheological and thermalbehavior of the exposed areas. Photocrosslinkable and photodimerizablecompositions are included in the definition of photohardenable.

A flexible, photohardenable element 1 as described above having aremovable cover sheet is continuously unwound from a storage roll 2 andis supported by means of a roll 3 as it passes a source of actinicradiation 4 whereby the element is imagewise exposed. The exposedelement passes from the exposure source into a source of particulatemetal 5, the cover sheet preferably present, is removed on a take uproll 6 prior to entering the particulate metal source 5. The particulatemetal adheres to the tacky nonexposed image areas of the photohardenedelement. From source 5 the metallized element moves past a heater 7,e.g., infrared heater, which heat sets the metal bearing or metalcontaining areas. The excess metal particles adhering to areas of theelement other than the tacky image areas are then removed by passing theheat set element through a water impingement cleaning device 8 which hascontainer 9 to contain the water spray. Brushes or other removal meansmay be present to help remove the excess toner. The element is movedpast an actinic radiation exposure source 10 which cures thephotohardenable layer thereby fixing the particulate metal to the layer.A conductive circuit is formed on the metal-bearing areas of the elementby passing the element into an electroless plating bath 11. The elementpasses through the plating bath over a series of rolls 12 which enablethe element to remain in the bath for a sufficient time period, e.g., 1to 6 hours, preferably 1 to 2 hours, to provide a conductive circuit ofthe desired thickness range, e.g., about 0.00025 to 0.001 inch (0.006 to0.025 mm) in thickness. In the FIGURE, two photohardenable elementsundergo simultaneous treatments as described above. After theelectroless plating, an adhesive layer from adhesive source 13 isapplied to both sides of a dielectric spacer 14 with openings in theswitch areas, and the two circuits and spacer are placed in register byregister means 15 and are laminated front to front by means of a heatpressure source 16. The laminate can then be cut to size, e.g., by a die17. Subsequently connectors can be applied to connect the circuits andafter testing for circuit reliability the printed circuit can bepackaged. It is understood that for many applications only a singleconductive circuit is applied to a flexible support thus the duplicateapparatus and dielectric spacer are not required. A single flexibleconductive circuit can also be adhered, e.g., laminated to a backingsheet, e.g., a rigid substrate or surface, e.g., a circuit board.

The flexible photohardenable elements as described above are critical tothe operation of the process of this invention. A flexible layer of aphotohardenable composition is formed on a flexible support. Preferablya flexible cover sheet is applied to protect the tacky surface of thephotohardenable layer, e.g., during storage. The cover sheet is notabsolutely necessary, however.

The type of photohardenable composition used is extremely importantsince in layer form the surface must be tacky and receptive toparticulate metal and must be able to withstand the rigors of the otherprocess steps including the electroless plating or soldering as well asany heating and curing steps. In addition, it is critical that theflexibility of the layer not be affected by the process of preparing theflexible circuit. Useful photohardenable compositions includephotopolymerizable compositions capable of addition polymerization whichare preferred, as well as photocrosslinkable and photodimerizablecompositions. Photopolymerizable compositions generally contain at leastone of the below-listed monomers, preferably addition polymerizableethylenically unsaturated compounds having at least two terminalethylenic groups, free radical generating addition polymerizationinitiators or initiator systems activatable by actinic radiation and atleast one organic elastomeric polymeric binder present in a predominantamount. Other components noted below can also be present in thecompositions. The monomer is present in 10 to 35 parts by weight,preferably 15 to 25 parts by weight; the initiator is present in 1 to 8parts by weight, preferably 3 to 5 parts by weight; and the elastomericbinder in 85 to 50 parts by weight, all based on the total weight of thecomposition. In layer form the composition is preferably 0.0005 to 0.006inch (0.013 to 0.152 mm) in thickness.

Monomers: t-butyl acrylate, 1,5-pentanediol diacrylate,N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate,1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethyleneglycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycoldiacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanedioldiacrylate, 2,2-dimethylol propane diacrylate, glycerol diacrylate,tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate,2,2-di(p-hydroxyphenyl)-propane diacrylate, pentaerythritoltetraacrylate, 2,2-di(p-hydroxyphenyl)-propane dimethacrylate,triethylene glycol diacrylate,polyoxyethyl-2,2-di(p-hydroxyphenyl)-propane dimethacrylate,di-(3-methacryloxy-2-hydroxypropyl) ether of Bisphenol-A,di-(2-methacryloxyethyl) ether of Bisphenol-A,di-(3-acryloxy-2-hydroxypropyl) ether of Bisphenol-A,di-(2-acryloxyethyl) ether of Bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-Bisphenol-A,di-(2-methacryloxyethyl) ether of tetrachloro-Bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-Bisphenol-A,di-(2-methacryloxyethyl) ether of tetrabromo-Bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol,di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid,triethylene glycol dimethacrylate, polyoxypropyltrimethylol propanetriacrylate (462), ethylene glycol dimethacrylate, butylene glycoldimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetrioltrimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate,pentaerythritol trimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate,pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate,1,5-pentanediol dimethacrylate, diallyl fumarate, styrene,1,4-benzenediol dimethacrylate, 1,4-diisopropenyl benzene, and1,3,5-triisopropenyl benzene.

In addition to the ethylenically unsaturated monomers mentioned above,the photohardenable layer can also contain at least one of the followingfree-radical initiated, chain-propagating, addition polymerizable,ethylenically unsaturated compounds having a molecular weight of atleast 300. These monomers include, preferably, an alkylene or apolyalkylene glycol diacrylate prepared from an alkylene glycol of 2 to15 carbons or a polyalkylene ether glycol of 1 to 10 ether linkages, andthose disclosed in U.S. Pat. No. 2,927,022, which is incorporated byreference, e.g., those having a plurality of addition polymerizableethylenic linkages, particularly when present as terminal linkages, andespecially those wherein at least one and preferably most of suchlinkages are conjugated with a double bonded carbon, including carbondoubly bonded to carbon and to such hetero atoms as nitrogen, oxygen andsulfur. Outstanding are such materials wherein the ethylenicallyunsaturated groups, especially the vinylidene groups, are conjugatedwith ester or amide structures.

Initiators: substituted or unsubstituted polynuclear quinones which arecompounds having two intracyclic carbon atoms in a conjugatedcarbocyclic ring system. Suitable such initiators include9,10-anthraquinone, 1-chloroanthraquinone, 2-chloroanthraquinone,2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone,octamethylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone,1,2-benzanthraquinone, 2,3-benzanthraquinone,2-methyl-1,4-naphthoquinone, 2,3-dichloroanaphthoquinone,1,4-dimethylanthraquinone, 2,3-dimethylanthraquinone,2-phenylanthraquinone, 2,3-diphenylanthraquinone, sodium salt ofanthraquinone alphasulfonic acid, 3-chloro-2-methylanthraquinone,retenequinone, 7,8,9,10-tetrahydronaphthacenequinone, and1,2,3,4-tetrahydrobenz(a) anthracene-7,12-dione. Other photoinitiatorswhich are also useful, even though some may be thermally active attemperatures as low as 85° C., are described in Plambeck U.S. Pat. No.2,760,863 and include vincinal ketaldonyl compounds, such as diacetyl,benzil, etc.; α-ketaldonyl alcohols, such as benzoin, pivaloin, etc.;acyloin ethers, e.g., benzoin methyl and ethyl ethers, etc;α-hydrocarbon substituted aromatic acyloins, including α-methylbenzoin,α-allylbenzoin and α-phenylbenzoin. In addition the photoreducible dyesand reducing agents disclosed in U.S. Pat. Nos. 2,850,445; 2,875,047;3,097,096; 3,074,974; 3,097,097; and 3,145,104 as well as dyes of thephenazine, oxazine, and quinone classes may be used. Other suitablepolymerization initiators are Michler's Ketone, benzophenone2,4,5-triphenylimidazolyl dimers with hydrogen doners, and mixturesthereof as described in U.S. Pat. Nos. 3,427,161; 3,479,185 and3,549,367.

All the aforementioned U.S. Patents are incorporated by reference.

Elastomeric Binders: synthetic rubbers, e.g., butadiene/acrylonitrile,acrylonitrile/butadiene (carboxy-modified, e.g., 3%)acrylonitrile/butadiene/styrene, alkyl (1 to 4 carbon atoms)methacrylate/acrylonitrile/butadiene, alkyl (1 to 4 carbon atoms)methacrylate/styrene/acrylonitrile/butadiene interpolymers,2-chlorobutadiene/1,3-polymers, chlorinated rubber,styrene/butadiene/styrene, styrene/isoprene/styrene block copolymers andother block copolymers described by Holden et al. in U.S. Pat. No.3,265,765 which is incorporated by reference, n-butyl methacrylate,polyether polyurethane resin, etc. The elastomeric binders can be usedindividually or in combination or in combination with minor amounts ofnonelastomeric binders, e.g., polyacrylate and alpha-alkyl polyacrylateesters, e.g., polymethyl methacrylate and polyethylmethacrylate;polyvinyl esters, e.g., polyvinyl acetate, polyvinyl acetate/acrylate,polyvinyl acetate/methacrylate and hydrolyzed polyvinyl acetate;ethylene/vinyl acetate copolymers; polystyrenes; vinylidene chloridecopolymers, e.g., vinylidene chloride/acrylonitrile, vinylidenechloride/methacrylate and vinylidene chloride/vinyl acetate copolymers;polyvinyl chloride and copolymers, e.g., polyvinyl chloride/acetate;high molecular weight polyethylene oxides of polyglycols having averagemolecular weights of 4000 to 1,000,000; epoxides; copolyesters, e.g.,those prepared from the reaction product of a polymethylene glycol ofthe formula HO(CH₂)_(n) OH, where n is a whole number 2 to 10 inclusive,and (1) hexahydroterephthalic, sebacic and terephthalic acids, (2)terephthalic, isophthalic and sebacic acids, (3) terephthalic andsebacic acids, (4) terephthalic and isophthalic acids, and (5) mixturesof copolyesters prepared from said glycols and (i) terephthalic,isophthalic and sebacic acids and (ii) terephthalic, isophthalic,sebacic and adipic acids; nylons or polyamides, e.g., N-methoxymethylpolyhexamethylene adipamide; cellulose esters, e.g., cellulose acetate,cellulose acetate succinate and cellulose acetate butyrate; celluloseethers, e.g., methyl cellulose, ethyl cellulose and benzyl cellulose,polycarbonates; polyvinyl acetal, e.g., polyvinyl butyral, polyvinylformal; polyformaldehydes.

Other optional additives include: inhibitors, dyes, pigments,plasticizers, etc.

Suitable thermal polymerization inhibitors include p-methoxyphenol,hydroquinone, and alkyl and aryl-substituted hydroquinones and quinones,tert-butyl catechol, pyrogallol, copper resinate, naphthylamines,beta-naphthol, cuprous chloride, 2,6-di-tert-butyl-p-cresol,phenothiazine, pyridine, nitroso dimers, e.g.,1,4,4-tri-methyl-2,3,-diazobicyclo-[3.2.2]-non-2-ene-2,3-dioxide,dinitrobenzene, p-toluquinone and chloranil.

Various dyes may be added to increase the visibility of the imageformed. Pigments may also be used in this capacity. Any colorant used,however, should preferably be transparent to the actinic radiation used.

Useful plasticizers may be the monomer itself, e.g., a diacrylate ester,or any of the common plasticizers which are compatible with thepolymeric binder, e.g., dialkyl phthalate, polyethylene glycol, andalkyl phosphates.

Suitable flexible supports which bear the photohardenable layer include:polyethylene terephthalate, flame-treated polyethylene terephthalate(preferred), electrostatic discharge treated polyethylene terephthalate,polyimides, polyolefins, e.g., polypropylene; polyparabanic acid, etc.

Suitable removable cover sheets that may be present are: siliconetreated polypropylene (preferred), polyethylene, polyethyleneterephthalate, etc.

The element in roll form, e.g., wound around a storage roll, is utilizedin the process shown in the FIGURE and is necessary for the continuousprocess embodiment. The element can be present and processed inindividual steps, however, but this is less preferred since theoperation would be lengthy and more expensive. The more detaileddescription of the process that follows will be exemplified by the useof the element in roll form in a continuous process.

The photohardenable element is unwound from the roll and moved by meansof a roll or other drive means to an exposure source, i.e., an actinicradiation source, whereby the element is exposed imagewise and tacky(adherent) image or images are defined. Suitable radiation sourcesdepend on the photohardenable composition type. Generally, however,radiation sources that are rich in ultraviolet radiation are useful.Radiation sources are disclosed in U.S. Pat. Nos. 2,760,863, 3,649,268,and 4,157,407, the disclosures of which are incorporated by reference.The exposure may be through a phototool, negative or positive, havingthe circuit image including circuit trace. Special phototools may haveto be used for particular needs such as crossovers or the formation ofcircuits useful in multilayer circuits.

Ductile metal and alloy particles, plating catalyst particles, orcombinations thereof are applied to the imagewise exposed element. Theparticles adhere primarily to the tacky nonexposed image areas. Suitableparticles which can be electrolessly plated or soldered include: copper(preferred), nickel, tin, lead, solder, mixtures of copper and solder,copper-tin alloy, tin-lead alloy, aluminum, gold, silver, metal oxidessuch as titanium oxide, copper oxide, etc. The particles have an averagediameter of about 0.1 to 250 μm, preferably 1 to 10 μm. Mixed particlesizes can be used.

The particles can be applied to the tacky (unexposed) image areas inknown manner. A preferred application method is by use of a fluidizedbed of particles, such as described in Research Disclosure, June 1977,No. 15882 by Peiffer and Woodruff which is incorporated by reference.Other application or toning means for the particulate metals aredescribed in U.S. Pat. Nos. 3,060,024; 3,391,455; 3,506,483; 3,637,385;3,649,268; and 4,019,821, all of which are incorporated by reference. Itis preferred that the particulate metal is applied to the unexposedareas by imbedding the metal into the surface.

To improve the retention of the metal particles in the tacky image areaspreferably the photohardenable layer of the element is heat set bypassing it under or through a heating unit. A type of unit is aninfrared heater such as a Cal Ray® model by General Electric Co.,Schenectady, N.Y. The element is maintained in the presence of theheater for about 0.1 to 5 minutes.

It is important that any excess metal or alloy particles be removed fromthe nontacky image areas. Suitable mechanical and other means forremoving excess particles are described in the above-mentioned ResearchDisclosure. Preferably a water wash, e.g., by impingement jet, is usedwith gentle mechanical scrubbing. Air impingement devices are alsouseful as well as wiping means although care must be taken not to removemetal particles in the tacky image areas. The removal of excessparticles occurs subsequent to the heat setting if this step is used.

Again it is preferred that adhesion of the particulate metal to thephotohardened elements be improved. The particulate metal can be fixedor bound to the photohardened layer by heating, e.g., in the temperaturerange of 80° to 150° C. for at least 10 seconds, the temperature beingbelow the degradation temperature of the photohardened element; cured byexposure to actinic radiation, e.g., by means of an actinic radiationsource as described above, or mechanical embedding, e.g., passing thetoned circuit through the nips of mechanical rollers. Combinations ofthese means also can be used. The preferred fixing means is by exposureto actinic radiation as shown in the drawing. Generally this exposuretime is in the range of 0.5 to 10 minutes.

Electroless plating procedures are known to those skilled in the art,e.g., U.S. Pat. No. 4,054,483 which is incorporated by reference.Electroless plating baths are commercially available, e.g., from theShipley Company, Revere, MA, Kollmorgen Corp., Glen Cove, N.Y., andother sources. Such baths may have to be modified to insure that themetal-bearing flexible element is maintained in the bath for asufficient period of time, e.g., 1 to 6 hours, preferably 1 to 2 hours.Useful electroless plating solutions are described in the examplesbelow, Zeblisky et al. U.S. Pat. No. 3,095,309, particularly Example II,which is incorporated by reference. The temperature of the electrolessplating bath can range from 43° to 65° C., preferably 50° C.

The circuit can also be formed by soldering techniques as known to thoseskilled in the art, e.g., Cohen and Peiffer, Published EuropeanApplication No. 0.003363 published Mar. 8, 1979 which is incorporated byreference. Prior to soldering, the metallized areas are treated by knownprocedures with solder fluxes typified by the following compositions:

Rosin type, organic, e.g., a mixture of diterpene acids in alcohol,water, or other appropriate solvent;

Inorganic acids, e.g., HCl, orthophosphoric acid, etc.;

Inorganic salts, e.g., zinc chloride, ammonium chloride and combinationsthereof used to produce HCl upon heating in the presence of water;

Organic acids, e.g., lactic, citric, oleic, etc.;

Organic halogen compounds, e.g., aniline hydrochloride, etc.;

Organic amines and amides, e.g., urea, etc.

These fluxes can contain curing or hardening catalysts or reagents.

The fluxed image areas are subsequently exposed to molten solder, e.g.,solder waves, etc. Known solder compositions can be used, e.g., tin,lead combinations, and compositions containing bismuth, cadmium, indium,silver and antimony. When the adherent material is thermally stable, lowmelting metals alone may be used, e.g., tin, lead, indium, etc.Preferred solders particularly useful in the preparation of printedcircuits are tin/lead in ratios of 63/37 and 60/40. It is important tobalance the ability to solder with the adhesion of the metal particlesin the adherent image areas, e.g., the areas should be tacky enough toadhere the metal particles but not too tacky whereby the particles wouldbecome coated or engulfed with the tacky material.

Once a conductive circuit has been formed as described above the circuitcan be electroplated by standard electroplating procedures to increasethe thickness of the circuit.

Two flexible circuits prepared as described above prepared eitherseparately or simultaneously (as shown in the Figure) can be used inconjunction with one another to form circuits. One useful form is amembrane switch wherein the flexible circuits are placed in registerface to face and are adhesively joined together, e.g., by lamination atelevated temperature, separated by a perforated dielectric layer whichallows contact at desired points when mechanical pressure is applied toone side of the circuit. A membrane switch can also be prepared from asingle flexible circuit as follows: (1) folding 180° the electricallyconductive circuit having first and second sections having contactorsand contacts, respectively, whereby the contactors and contacts areadjacent to each other, and (2) inserting between said folded conductivecircuit a dielectric spacer having a plurality of openings therethroughfor selective alignment with the contacts and contactors.

Flexible multilayer circuits with conductive interconnectors can beprepared in the manner generally described in Example 8 below using aflexible substrate and a layer or layers of the flexible photohardenablecomposition as described in this invention. Additional information aboutthe preparation of multilayer circuits with conductive interconnectionsis found in U.S. Pat. No. 4,157,407, which is incorporated by reference.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for electroless plating is illustrated in Examples 1 and2, the latter for an automated procedure. Example 6 illustrates the bestmode for soldering to form the flexible circuit.

INDUSTRIAL APPLICABILITY

The process of this invention is easily adapted to automation and isapplicable to the preparation of fine conductive wiring traces and lowcost flexible printed circuits including multilayer printed circuits.The printed circuit obtained by the process can be laminated to a rigidbacking material for use as a rigid membrane switch. A flexible circuitcan be laminated to a ground plane or to a rigid circuit to form amultilayer circuit board. A preferred use for the flexible printedcircuit is in the preparation of membrane switches where flexure isneeded to provide mechanical ability to open and close contacts. Suchswitches are formed as described in the FIGURE by applying two formedcircuits in register to a suitable dielectric sheet or film having holescorresponding to the switch contacts. Alternatively, the switches can beformed by folding the formed circuit in register and inserting asuitably sized dielectric sheet or film between the fold. The membraneswitch is useful as a telephone keyboard, in electrical appliances,electrical toys, calculators, or other objects which require a magnitudeof depressions, e.g., millions, throughout their useful lives.

EXAMPLES

The following examples illustrate the invention wherein the parts andpercentages are by weight. In Examples 1 to 5 membrane switches areprepared using flexible photoadhesive layers on a flexible substrate andelectroless plating.

EXAMPLE 1

A methylene chloride solution (82%) of a tacky photopolymer compositionis coated on each of two flame-treated polyethylene terephthalate films,0.005 inch (0.127 mm) thick, and is dried [0.0016 inch (0.041 mm)]. Thephotopolymer composition is prepared from the following ingredients:

    ______________________________________                                        Ingredient            Amount (Parts)                                          ______________________________________                                        Rubber [3% carboxy-modified                                                                         58.0                                                    acrylonitrile(27)/butadiene(73)],                                             high molecular weight, average                                                Mooney viscosity is 45                                                        Methyl methacrylate(34)/styrene(42)/                                                                12.5                                                    acrylonitrile(8)/butadiene(16)                                                interpolymer                                                                  Trimethylolpropane triacrylate                                                                      25.0                                                    Benzophenone          4.0                                                     4,4-Bis(dimethylamino)benzophenone                                                                  0.5                                                     1,4,4-trimethyl-2,3,-diazobicyclo-                                                                  0.05                                                    [3.2.2]non-2-ene-2,3,-dioxide                                                 ______________________________________                                    

A silicone release coated polyethylene terephthalate film cover sheet,0.001 inch (0.0254 mm) thick, is laminated to the dry photopolymer layergiving flexible support-photopolymer layer-cover sheet sandwichs(elements).

A positive transparency of a membrane switch circuit (phototool) isplaced in contact with the cover sheet side of the element, and theelement is imagewise exposed for 30 units with, a 5 Kw actinic radiationsource using a Riston® PC 24 Printer, E. I. du Pont de Nemours andCompany, Wilmington, DE. The cover sheet and phototool are removed, andthe element is toned in a fluid bed toner with 8 μm copper powder,Alcan® MD-301. Copper powder adheres to the unexposed areas. The tonedcircuit is heated for 3 minutes in a 150° C. oven to heat set the tonedareas. Excess copper powder is removed with water and mild abrasion. Themetallized element is cured by exposing to ultraviolet radiation in anArgus ultraviolet processor model PC7100 for 15 seconds. The tonedcircuit is then plated in a Shipley electroless copper plating bath,Cuposit® CP-74, Shipley Co., Revere, MA. for one hour at 50° C. Afterplating, the circuit elements are registered and with a hole puncheddielectric separation sheet are assembled as a membrane switch.

The resulting membrane switch has a resistance of less than 2 ohms inthe closed position. The switch is repeatedly wrapped around a dowel,0.375 inch (about 9.53 mm) in diameter without showing physical orelectrical damage. The circuit elements are firmly attached to thepolyethylene terephthalate substrate and cannot be removed by applyingScotch® Magic Mending Tape and stripping the tape.

EXAMPLE 2

This example illustrates the preparation of a membrane switch in anautomated production facility.

A roll of the photopolymer element as described in Example 1 is attachedto a polyethylene terephthalate film 0.005 inch (0.127 mm) in thicknessthat is threaded the entire length of the production line. Positiveimage-bearing phototools of the switch pattern are placed on the coversheet of the element as it is drawn at 6 inches/minute (15.24 cm/min.)under an Oriel® collimated light source. The exposure time is 20 secondsat 6.5 milliwatts/cm². After exposure the phototool and element coversheet are removed automatically and the exposed element is driven overrollers through a fluid bed toner apparatus (Research Disclosure No.15882, Peiffer and Woodruff, June 1977) containing about 8 μm copperparticles for about 30 seconds. The toned element is transported undertwo infrared heaters and is heated to a temperature of about 85° C.which heat sets the toned areas. The heat set element is then drivenpast a cleaning unit where it is spray-washed with water and gentlyabraded with counter-rotating pads. The toned element then passes undera source of actinic radiation where it is exposed for one minute at 7.8mw/cm² to cure the photopolymer layer and to firmly anchor the copperparticles. Optionally air can be excluded to improve the curing step.The cured element continues into an aqueous solution of ammoniumpersulfate (10%) for one minute and is rinsed with water for one minute.The element transport speed is slowed to 1.5 inches/minute (3.81cm/min.) as it travels through an electroless copper plating solution(Add Plate®480, PCK Technology Division of Kollmorgen Corp., Glen Cove,N.Y.) for one hour. The plating solution is maintained at 53° C. in a J.Holland and Sons, Inc. laboratory electroless copper plating module. Asthe plated circuit element emerges from the plating bath, it is rinsedin water, dried and wound up on a rewind mandrel. Other circuits followas the sequence is repeated. The circuit pattern is then cut, isassembled as a membrane switch, and is tested. The switch has the sameproperties as the switch prepared as described in Example 1.

EXAMPLE 3

Example 1 is repeated except that the photopolymer layer having a drythickness of 0.0009 inch (0.0229 mm) is prepared from the followingingredients:

    ______________________________________                                        Ingredient            Amount (Parts)                                          ______________________________________                                        Polyether polyurethane resin.sup.1                                                                  69.0                                                    Trimethylolpropane triacrylate                                                                      25.0                                                    4,4'-Dichlorobenzophenone                                                                           5.5                                                     4,4'-Bis(dimethylamino)benzophenone                                                                 0.5                                                     ______________________________________                                         .sup.1 The polyether polyurethane resin, Qthane ® P2502, a product of     K. J. Quinn Co., Malden, MA, is a crystalline, thermoplastic resin having     a Brookfield viscosity of 600 to 1200 centipoises using 15% solids in         methyl ethyl ketone and a Brookfield spindle #3 at 12 rpm and an adhesive     activation temperature of about 63° C., and a decrystallization        temperature of about 49° C.                                       

The resulting membrane switch has a resistance of less than 2 ohms inthe closed position and is wound around a dowel, 0.375 inch (about 9.53mm) in diameter without damage. The anchorage is satisfactory as shownby the tape test described in Example 1.

EXAMPLE 4

Example 1 is repeated except that the photopolymer layer having a drythickness of 0.0007 inch (0.0178 mm) is prepared from the followingingredients:

    ______________________________________                                        Ingredient            Amount (Parts)                                          ______________________________________                                        Interpolymer described in Example 1                                                                 64.0                                                    Trimethylolpropane triacrylate                                                                      29.0                                                    4,4'-Dichlorobenzophenone                                                                           6.4                                                     4,4'-Bis(dimethylamino)benzophenone                                                                 0.6                                                     ______________________________________                                    

The resulting membrane switch has a resistance of less than 2 ohms inthe closed position. No electrical or physical damage is noted when theswitch is wrapped around a dowel as described in the previous examples.The anchorage is good as shown by the tape test described in Example 1.

EXAMPLE 5

Example 1 is repeated except that the photopolymer layer having a drythickness of 0.0007 inch (0.0178 mm) is prepared from the followingingredients:

    ______________________________________                                        Ingredient            Amount (Parts)                                          ______________________________________                                        n-Butylmethacrylate polymer,                                                                        68.4                                                    inherent viscosity of 0.25 g                                                  polymer in 50 ml CH.sub.2 Cl.sub.2                                            measured at 20° C. using a No. 50                                      Cannon-Fenske Viscometer is 0.5                                               Trimethylolpropane triacrylate                                                                      21.4                                                    Tris(4-diethylamino-o-tolyl) methane                                                                5.1                                                     4,4'-Dichlorobenzophenone                                                                           4.7                                                     4,4'-Bis(dimethylamino)benzophenone                                                                 0.4                                                     ______________________________________                                    

The resulting membrane switch has a resistance of less than 2 ohms inthe closed position. No electrical or physical damage is noted when theswitch is wrapped around a dowel as described in Examples 1 and 3. Theanchorage is good as shown by the tape test described in Example 1.

EXAMPLE 6

A layer of a photopolymer composition prepared as described in Example1, 0.0011 inch (0.028 mm) in thickness supported on a 0.002 inch (0.051mm) polyimide film and bearing a 0.003 inch (0.076 mm) silicone releaselayer coated polypropylene cover sheet is exposed to a positive circuitpattern for 15 seconds on an ultraviolet exposure source, a 400 watt,medium pressure, mercury vapor lamp on a Model DM VL-HP Double SidedExposure Frame, manufactured by Colight, Inc., Minneapolis, Minn. Thecover sheet is removed and the flexible exposed element is dipped into afluid bed toner containing 8 μm copper powder. The exposed element isfastened temporarily for support to a rigid sheet of glass epoxy boardcommonly used in the printed circuit industry, and the element is heatedin an oven at 150° C. for 50 seconds. After removal from the oven andcooling to room temperature, excess copper powder is removed with awater spray. The element is dried and is conveyed twice through an Argusultraviolet processor model PC7100 at 10 feet/minute (3.048 m/min.). Anaqueous solder flux, Alpha®709F manufactured by Alpha Metals, Inc.,Jersey City, N.J., is brushed on the metal-toned areas, and the fluxedelement is conveyed at 6 feet/minute (1.83 m/min.) through a wave-soldermachine manufactured by Hollis Engineering, Inc., Nashua, N.H. The tonedcircuit areas become covered with solder (tin/lead (60/40)). Theresistance of a 7 inch (17.78 cm) long 0.040 inch (1.02 mm) wide line isabout 0.2 ohm. After removal of the support film, the circuit isrepeatedly wrapped around a dowel, 1.5 inches (about 3.81 cm) indiameter without showing physical or electrical damage. The circuit isfirmly attached to the polyimide support and is not removed by the tapetest described in Example 1.

EXAMPLE 7

A photopolymer element as described in Example 6 without the cover sheetis exposed for 30 seconds to a positive of a printed circuit trace bymeans of the exposure device described in Example 6. The flexibleexposed element is dipped into a fluid bed toner containing 8 μm copperpowder. The element is heated in an oven at 120° C. for 2 minutes, iscooled to room temperature and is cleaned of excess particulate copperby means of a water spray. The element is then dried and is cured bypassing through the actinic radiation source as described in Example 6.The toned circuit trace is plated in a Shipley electroless copperplating bath, Shipley Co., Revere, MA, for 6 hours at 50° C. The platedelement is heated in an oven at 150° C. for 60 minutes and immediatelythe solder flux described in Example 6 is applied to the conductivecopper traces followed by passing the fluxed element through thewave-solder machine as described in Example 6. The resistance of thecircuit trace is 0.2 ohm. The circuit is flexible and is firmly attachedto the support after being tested as described in Example 1.

EXAMPLE 8

This example illustrates the preparation of flexible multilayercircuits.

A flexible circuit is prepared as described in Example 1. A layer of asimilar flexible photopolymer composition, 0.002 inch (0.051 mm) islaminated to the flexible circuit. The laminated element is exposed asdescribed in Example 1 using a phototool which leaves only the areasunexposed where circuit lines or "cross-over" lines are required. Theelement is toned in a fluid bed toner containing 8 μm copper powder, andthe toned circuit is heated for 2 minutes in an oven at about 150° C.Excess copper powder is removed as described in Example 1 and thelaminate is dried. The laminate is exposed a second time to the sameexposure source whereby all areas containing toner are polymerized withthe exception of the areas where interconnecting pathways are requiredbetween the circuit lines on the initial circuit and the "cross-over"lines on the second flexible photopolymer layer. The areas leftunpolymerized are masked by a second phototool defininginterconnections. The interconnecting unpolymerized areas are washed outusing methylchloroform. Additionally if contactors are located on theinitial circuit the corresponding areas of the second photopolymer layerare washed out with the solvent. The interconnections obtained are tonedwith copper powder as described previously above, the laminate is thenbaked in an oven at 150° C. for 2 minutes, the excess copper powder isremoved as described above, and is cured by exposing to ultravioletradiation in an Argus ultraviolet processor model PC7100 for 15 seconds.The toned circuit is electrolessly plated as described in Example 1forming a flexible multilayer circuit.

Control 1

Example 1 is repeated except that the tacky photohardenable layer havinga dry thickness of 0.002 inch (0.051 mm) is prepared from the followingingredients:

    ______________________________________                                        Ingredient             Amount (Parts)                                         ______________________________________                                        Interpolymer described in Example 1                                                                  25.4                                                   Methylmethacrylate (96%)/ethyl-                                                                      7.9                                                    methacrylate(4%)copolymer,                                                    inherent viscosity of 0.25 g                                                  polymer in 50 ml CH.sub.2 Cl.sub.2                                            measured at 20° C. using a No. 50                                      Cannon-Fenske Viscometer is 0.5                                               Pentaerythritol triacrylate                                                                          19.3                                                   Di-(3-acryloxy-2-hydroxypropyl)                                                                      19.3                                                   ether of bisphenol-A                                                          4,4'-Dichlorobenzophenone                                                                            4.6                                                    4,4'-Bis(dimethylamino)benzophenone                                                                  0.4                                                    Monastral Green ® (pigment 30%) roll-                                                            0.1                                                    mill blended with the interpolymer                                            described in Example 1                                                        Polyethylene covered talc, 5 μm                                                                   23.0                                                   ______________________________________                                    

The resulting membrane switch while having a resistance of less than 2ohms cannot be effectively wrapped around a dowel as described inExample 1. The photohardenable layer cracks and the adhesion to thepolyethylene terephthalate film fails.

Control 2

Control 1 is repeated except that the tacky photohardenable layer has adry thickness of 0.0011 inch (0.028 mm) is prepared from the followingingredients:

    ______________________________________                                        Ingredient            Amount (Parts)                                          ______________________________________                                        Polyester prepared by reacting                                                                      69.0                                                    1.0 mole ethylene glycol, 0.5                                                 mole azelaic acid, 0.17 mole                                                  isophthalic acid, and 0.33                                                    mole terephthalic acid, the                                                   no. average mol. wt. is about                                                 19,000 and the wt. average                                                    mol. wt. is about 37,000                                                      Trimethylolpropane triacrylate                                                                      25.0                                                    4,4'-Dichlorobenzophenone                                                                           5.5                                                     4,4'-Bis(dimethylamino)benzophenone                                                                 0.5                                                     ______________________________________                                    

The resulting membrane switch has a resistance greater than 10,000 ohms.The plating bath blisters the conductive circuit giving poor anchorageto the flexible support.

We claim:
 1. A process for the preparation of two flexible circuitsformed from flexible photohardenable elements which comprises:(a)exposing imagewise to actinic radiation each of the two flexiblephotohardenable elements comprising, in order,(1) a flexible support,(2) a dry, tacky photohardenable layer comprising a photohardenablecomposition, including at least one organic elastomeric polymeric bindercompound, and, optionally (3) a removable cover sheet, (b) removing thecover sheet from each element, if present, and applying particulatemetal to the unexposed image areas of each photohardenable layer;optionally (c) heating the elements to a temperature in the range of 25°C. to 80° C.; then (d) removing excess particulate metal; optionally (e)fixing or binding the particulate metal to each photohardenable layer byheating, curing by exposing to actinic radiation, embeddingmechanically, and combinations thereof; (f) plating electrolessly orsoldering the particulate metal bearing or metal containing areasthereby forming electrically conductive flexible circuits, saidelectrically conductive flexible circuits when having a thickness of0.001 inch can be repeatedly bent around a structure having a diameterof 0.600 inch without breaking, cracking or delaminating, saidelectrically conductive flexible circuits when having thicknessesdifferent from 0.001 inch, the diameter of said structure would beincreased or decreased proportionally with the proviso that the minimumdiameter of said structure would always be greater than 0.25 inch, (g)placing the two flexible circuits in register face to face, and (h)joining adhesively said two flexible circuits together, said circuitsbeing separated by a perforated dielectric layer which allows contact atdesired points when mechanical pressure is applied to one side of thecircuit.
 2. A process according to claim 1 wherein the joining isaccomplished by lamination at elevated temperature.
 3. A processaccording to claim 1 wherein the photohardenable layer comprises (i) anaddition polymerizable ethylenically unsaturated compound having atleast two terminal ethylenic groups, (ii) at least one organicelastomeric polymeric binder compound, and (iii) a free radicalgenerating addition polymerization initiator or initiator systemactivatable by actinic radiation.
 4. A process according to claim 1wherein the binder is a combination ofmethylmethacrylate/styrene/acrylonitrile/butadiene interpolymer and acarboxylated acrylonitrile/butadiene rubber compound.
 5. A processaccording to claim 1 wherein the binder is amethylmethacrylate/styrene/acrylonitrile/butadiene interpolymer.
 6. Aprocess according to claim 1 wherein the binder is a polymer ofn-butylmethacrylate.
 7. A process according to claim 1 wherein thebinder is a polyether polyurethane polymer.
 8. A process according toclaim 1 wherein the steps of the process are performed continuously. 9.A process according to claim 1 wherein the particulate metal is fixed orbound to the photohardened layer by heating in the temperature range of80° to 150° C., for at least 10 seconds, the temperature being below thedegradation temperature of the photohardened element.
 10. A processaccording to claim 1 wherein the particulate metal bearing areas areplated electrolessly.
 11. A process according to claim 1 wherein theparticulate metal bearing areas are soldered.
 12. A process for thepreparation of a membrane switch from a flexible circuit formed from aflexible photohardenable element which comprises(a) exposing imagewiseto actinic radiation a flexible photohardenable element comprising, inorder,(1) a flexible support, (2) a dry, tacky photohardenable layercomprising a photohardenable composition, including at least one organicelastomeric polymeric binder compound, and, optionally (3) a removablecover sheet, (b) removing the cover sheet, if present, and applyingparticulate metal to the unexposed image areas of the photohardenablelayer; optionally (c) heating the element to a temperature in the rangeof 25° C. to 80° C.; then (d) removing excess particulate metal;optionally (e) fixing or binding the particulate metal to thephotohardenable layer by heating, curing by exposing to actinicradiation, embedding mechanically, and combinations thereof; (f) platingelectrolessly or soldering the particulate metal bearing or metalcontaining areas thereby forming an electrically conductive flexiblecircuit, said electrically conductive flexible circuit when having athickness of 0.001 inch can be repeatedly bent around a structure havinga diameter of 0.600 inch without breaking, cracking or delaminating,said electrically conductive flexible circuit when having thicknessesdifferent from 0.001 inch, the diameter of said structure would beincreased or decreased proportionally with the proviso that the minimumdiameter of said structure would always be greater than 0.25 inch; (g)folding 180° the electrically conductive circuit having first and secondsections having contactors and contacts, respectively, whereby thecontactors and contacts are adjacent to each other, and (h) insertingbetween said folded circuit a dielectric spacer having a plurality ofopenings therethrough for selective alignment with the contacts andcontactors.
 13. A process according to claim 12 wherein thephotohardenable layer comprises (i) an addition polymerizableethylenically unsaturated compound having at least two terminalethylenic groups, (ii) at least one organic elastomeric polymeric bindercompound, and (iii) a free radical generating addition polymerizationinitiator or initiator system activatable by actinic radiation.
 14. Aprocess according to claim 12 wherein the binder is a combination ofmethylmethacrylate/styrene/acrylonitrile/butadiene interpolymer and acarboxylated acrylonitrile/butadiene rubber compound.
 15. A processaccording to claim 12 wherein the binder is amethylmethacrylate/styrene/acrylonitrile/butadiene interpolymer.
 16. Aprocess according to claim 12 wherein the binder is a polymer ofn-butylmethacrylate.
 17. A process according to claim 12 wherein thebinder is a polyether polyurethane polymer.
 18. A process according toclaim 12 wherein the particulate metal is fixed or bound to thephotohardened layer by heating in the temperature range of 80° to 150°C., for at least 10 seconds, the temperature being below the degradationtemperature of the photohardened element.
 19. A process according toclaim 12 wherein the particulate metal bearing areas are platedelectrolessly.
 20. A process according to claim 12 wherein theparticulate metal bearing areas are soldered.